Reference intervals for first trimester embryonic/fetal heart rate in a Thai population.

OBJECTIVE

To establish reference intervals for first trimester embryonic/fetal heart rate in a Thai population.

METHODS

A prospective descriptive cross-sectional study was performed on normal pregnant women registered at the antenatal clinic in their first trimester. All had a good menstrual history and the calculated gestational age using crown-rump length (CRL) in the first trimester ultrasound was in agreement. Crown-rump length and embryonic/fetal heart rate measurements were obtained by a 5 MHz vaginal probe combined with a duplex color Doppler machine. All of the newborns were proved to be normal at birth. We constructed and compared embryonic/fetal heart rate expressed as beats/min relating to gestational age (GA) (days) and CRL. The data was analyzed and the best fit mathematical model was derived using the SPSS computer program.

RESULTS

547 cases were enrolled into the study. The median embryonic/fetal heart rate increased from 124 beats/min at GA of 40-44 days to 177 beats/min at GA 60-64 days, thereafter, embryonic/fetal heart rate gradually decreased to 159 beats/min at GA 95-99 days. The best fit regression equation was: embryonic/fetal heart rate (beats/min) = -21.666 + 5.4796 GA (day) - 0.0383 GA (day)2, R = 0.5, p < 0.0001). In addition, embryonic/fetal heart rate also correlated with CRL. The median embryonic/fetal heart rate increased from 131 beats/min at CRL of 5-9 mm to 177 beats/min at CRL 20-34 mm, thereafter, embryonic/fetal heart rate gradually decreased to 155 beats/min at CRL 75-79 mm. The best fit regression equation was: embryonic/fetal heart rate (beats/min) = 127.361 + 3.4939 CRL (mm) - 0.0749 CRL (mm)2 + 0.0004 CRL (mm)3, R = 0.58, p < 0.0001).

CONCLUSION

Reference intervals for first trimester embryonic/fetal heart rate in a Thai population were established. Our reference intervals may be useful for further studies, such as the prediction of spontaneous abortion or chromosomal disorders after ultrasound-proven viability in the first trimester.